Field of the Invention
The present invention is directed to the use of inhibitors of the epithelial sodium channel (ENaC) for treating a variety of conditions affecting the skin.
Description of the Background
Skin is the physical barrier separating an organism and its environment, which prevents water loss and protects from chemical, mechanical, and microbial attacks. To perform these functions, the epidermis, as the outer layer of the skin, undergoes keratinization, a process in which epidermal cells mature from proliferative basal cells to the terminally differentiated cells of the stratum corneum. The differentiation of epidermal basal cells is caused by changes in protein and enzymes which regulate metabolic changes and alterations in lipid synthesis and composition.
Hydration is important to the process of epithelial wound healing, as healing that occurs in a wet environment, is faster and results in less scarring. Essential to the healing process are the ability of the epithelium to: (1) re-establish the water barrier and (2) reduce inflammatory cytokine expression. Recent studies suggest that reduced hydration upon disruption of stratum corneum can cause an ion flux of epithelial cells and that the reduction of hydration caused by skin barrier disruption leads to greater changes in local ion concentrations. As sodium is the most abundant cation in skin extracellular matrix, it is believed to be involved in keratinocyte differentiation and normal epidermal growth. The epithelial sodium channel (ENaC) has been described as an important regulator of epidermal homeostasis and wound healing, both involved in epidermal “sensing” of the water barrier function and inflammatory pathways associated with scarring.
ENaC is a member of the ENaC/degenerin family of ion channels. They are highly Na+ selective channels that are comprised of three structurally related subunits (α, β and γ) that share a similar secondary structure consisting of an extracellular region linked to two transmembrane domains. In some tissues, a fourth δ-ENaC subunit may be expressed, resulting in the formation of channels with distinct biophysical characteristics. The resolved structure of a related channel, the acid sensing ion channel (ASIC1), has provided insights into the structural organization of the ENaCs, suggesting that functional channels are heterotrimeric subunits.
ENaCs mediate Na+ transport across apical or luminal membranes, providing the rate-limiting step of transepithelial Na+ uptake. ENaCs are expressed in many salt-reabsorbing epithelia, including the renal distal nephron, airway, and colon. ENaC-mediated Na+ absorption in the distal nephron has an essential role in extracellular volume homeostasis and blood pressure regulation, while Na+ absorption in the airway has a key role in regulating airway surface liquid volume and the rate of mucus transport.
In the epidermis, keratinocytes express amiloride-sensitive ENaCs which has been demonstrated to be required for normal barrier function. Systemic genetic depletion of α-ENaC in mice has been shown to disrupt the formation of then normal skin barrier, highlighting the importance of ENaC in the epidermis. Furthermore, ENaC-mediated sodium flux in keratinocytes increases the secretion of inflammatory cytokines via the COX-2/prostaglandin E2 (PGE2) pathway (Charles, R.-P., Guitard, M., Leyvraz, C., Breiden, B., Haftek, M., Haftek-Terreau, Z., Hummler, E. (2008). Postnatal requirement of the epithelial sodium channel for maintenance of epidermal barrier function. The Journal of Biological Chemistry, 283(5), 2622-30). The role of ENaC as an upstream mediator of prostaglandin E2 release has been further confirmed in the uterine endometrium, where ENaC is required for embryo implantation. (Ruan, Y. C., Guo, J. H., Liu, X., Zhang, R., Tsang, L. L., Dong, J. Da, Chan, H. C. (2012). Activation of the epithelial Na+ channel triggers prostaglandin E2 release and production required for embryo implantation. Nature Medicine, 18(7), 1112-7). As such, the inhibition of ENaC in keratinocytes has been proposed to promote healing of the skin via multiple processes that may include changes in keratinocyte differentiation, proliferation, and inflammatory signaling. (Xu, W., Hong, S. J., Zeitchek, M., Cooper, G., Jia, S., Xie, P., Mustoe, T. (2014). Hydration Status Regulates Sodium Flux and Inflammatory Pathways through Epithelial Sodium Channel (ENaC) in Skin. The Journal of Investigative Dermatology, (August), 1-26; Maubec, E., Laouénan, C., Deschamps, L., Nguyen, V. T., Scheer-Senyarich, I., Wackenheim-Jacobs, A.-C., Farman, N. (2015). Topical Mineralocorticoid Receptor Blockade Limits Glucocorticoid-Induced Epidermal Atrophy in human Skin. The Journal of Investigative Dermatology, (February). Doi:10.1038; Sharma, R. K., Gupta, B., & Sharma, B. (2014). Original Article Topical amiloride delays healing of deep thermal wounds in albino rabbits, 58(3), 251-261; Sharma, R. K., Gupta, B., & Sharma, B. (2014). Original Article Topical amiloride delays healing of deep thermal wounds in albino rabbits, 58(3), 251-261.).
The inhibition of ENaC mediated Na+ transport and downstream signaling pathways can be accomplished with an ENaC blocker of the amiloride class (which blocks from the extracellular domain of ENaC). In dermal wound healing studies in animals and man, topical application of amiloride has been shown to accelerate the rate of wound healing. These findings are consistent with the role of ENaC in epidermal hydration and local inflammatory processes. Importantly, ENaC inhibitors must be maintained on the extracellular surface of the target tissue, at the site of the channel, to achieve and maintain the therapeutic utility.
The present invention describes conditions in which the inhibition of ENaC promotes hydration of the skin and/or prevents inflammatory signaling cascades in the skin in order to facilitate healing. Furthermore, the present invention describes ENaC inhibitors with increased potency, reduced cellular absorption, and slow dissociation (“unbinding” or detachment) from ENaC that are required for the therapy of skin conditions